The term rheology, describes the flow dynamics of liquids and the deformation of solids. Rheological properties of heterogeneous dispersions are complex and cannot be expressed in a single parameter. Manufacturers of medicinal and cosmetic gels, creams, pastes, lotions and foams must be capable of producing products with acceptable consistency and smoothness and reproducing these qualities each time a new batch is prepared including its look, feel, body, and consistency.
Rheology is involved in the mixing and flow of materials, their packaging into containers, and their dispensing prior to use, whether this is achieved by pouring from a bottle, extrusion from a tube, passage through a syringe needle, or extrusion through a valve. The rheology of a particular product, which can range in consistency from fluid to semisolid to solid, can affect its patient acceptability, physical stability, and even biologic availability. For example, viscosity which is a rheological property has been shown to affect absorption rates of drugs.
Pharmaceutical areas, in which rheology is significant include product design and processing are fluids, quasi-solids, solids, and processing. Rheology of fluids is pertinent in respect of: (a) mixing fluids; (b) reduction of systems with shear; (c) passage through orifices, including pouring, packaging in bottles, passage through hypodermic needles and passage through valves; (d) fluid transfer, including pumping and flow through pipes; and (e) physical stability of disperse systems. Rheology of quasi-solids or semi-solids is pertinent including in respect of: (a) spreading and adherence on the skin; (b) removal from jars or extrusion from tubes; (c) capacity of solids to mix with miscible liquids; and (d) release of the drug from the base.
When classifying materials according to types of flow and deformation, it is customary to place them in one of two categories: Newtonian or non-Newtonian systems. The choice depends on whether or not their flow properties are in accord with Newton's law of flow. Newton recognized that the higher the viscosity of a liquid, the greater is the force per unit area (shearing stress) required to produce a certain rate of shear. Many fluid pharmaceutical products behave as Non-Newtonian systems.
Rheological properties such as tackiness or stickiness, “body,” “slip,” and “spreadability” are difficult to measure by means of conventional apparatus and, in fact, do not have precise meanings. Whereas, viscosity, yield value, thixotropy, and the other properties that contribute to the total consistency of non-Newtonian pharmaceuticals can be analyzed.
Viscosity is a measure of the internal resistance of a fluid to flow which is being deformed by either shear stress or tensile stress; the higher the viscosity, the greater is the resistance. Simple liquids can be described in terms of absolute viscosity. In everyday terms (and for fluids only), viscosity is “thickness” and may be thought of as an indication of fluid friction. Shear viscosity, describes the reaction to applied shear stress; in other words, it is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction, to velocity gradient.
Gels are jelly-like material that can have properties ranging from soft and fluid to hard and tough. Gels may be in liquid, semi-liquid or solid state. Solid gels are defined as a substantially diluted crosslinked system, which exhibits no flow when in the steady-state. By weight, gels are mostly liquid, yet they behave like semi-solids due to a three-dimensional crosslinked network of a solidifying, gelling or thickening agent within the liquid. It is the crosslinks within the fluid that give a gel its structure (hardness) and contribute to stickiness (tack). Depending on the amounts of gelling agents in a formulation the gel may be semi solid with some limited flowability, such that when the semi-solid gel is placed in a tube and is inclined horizontally from a vertical position it will slowly flow from the vertical towards the horizontal or it may be a liquid gel where the amount of gelling agents or gelling effect is lower such that the gel structure or connections are weaker or loose so that when placed in a tube and tilted from a vertical position to the horizontal the gel readily flows and adapts to the horizontal position. The rheological properties of gels at different surface temperatures can influence the release and bioabsorption of drugs therefrom.
It is a desirable property for composition for topical use to have controllable viscosity. High viscosity is required to avoid drips and runs for ease of application and improve suspending properties in order to avoid rapid sedimentation of non-dissolved active ingredients. Whereas, low viscosity is desirable to enable spreadability and good flow properties. The less viscous something is, the greater its ease of movement (fluidity). A delicate balance between these two attributes is sought out in accordance with the intended use of the compositions. This balance is difficult to attain as viscosity of a composition is influenced by different factors such as reaction or interaction between different components under different temperature and pressure conditions.
Thickening or solidifying agent or solidifying complexes are materials added to a composition which increase viscosity and retard sedimentation. The use of waxes, fatty alcohols, fatty acids and 12 hydroxy stearic acid, in solidifying oils is known. Pharmaceutical compositions having a netted framework, comprising an oil and beeswax as a gelling agent that form a film after application on a body surface are also known. Netted frameworks and/or films can be in certain circumstances be a disadvantage.
The addition of a fatty alcohol, or a fatty acid, or both to a liquid oil also gives rise to thixotropic properties (e.g., being semi-solid at rest and liquid upon application of shear forces thereto). This property enables application of a thixotropic mixture as a semi-solid state to a body surface, which subsequently becomes substantially liquid and therefore more spreadable and penetrable when rubbed onto the body surface. Thus, they are semi-solid at rest and that they liquefy upon application of shear forces thereto. Semi-solid hydrophobic formulations are important not only for the pharmaceutical market but also for cosmetic products, such as carriers of sunscreen compounds, oil-soluble plant extracts, materials for scrubbing purposes and other active and non-active cosmetic ingredients.
Foams and, in particular, single-phase oleaginous foams are complicated systems which do not form under all circumstances. Slight shifts in foam composition, such as by the addition of active agents or the removal of any of the essential ingredients, may destabilize the foam. The prior art teaches that oleaginous foam compositions require significant amounts of surface active agents to form a foam. These compositions require various standard surfactants, as essential components.
Surfactants are known as essential ingredients in foam compositions; and specifically in oleaginous foams. However, many surfactants are known to be irritating when left on the skin can also react with unstable active agents and lead to their rapid degradation.
Gels and foams are not pharmaceutically equivalent, unless their composition upon administration is similar.